1. Field of the Invention
The present invention relates to an electronic still camera.
2. Related Background Art
In a video camera or an electronic still camera, the image is photoelectrically converted into an electrical signal, which is recorded after signal processing such as amplification, modulation etc. In the amplification of the electrical signal, a change in the gain of amplification is equivalent to a change in the sensitivity of the camera. A faster shutter speed achieved by an increased photographing sensitivity of the camera is effective for preventing the undesirable effect of camera vibration or for photographing a moving object, and a camera with sensitivity increased by such method is already known.
However, if the gain is increased in the processing circuit for the output signal of a solid-state image sensor for photoelectric conversion of the image, for the purpose of equivalent increase of the sensitivity of the image sensor (hereinafter called sensitivity increase), noise is also amplified together with the image signal, thus reducing the S/N ratio of the image signal.
A major cause of such noise is the dark current of the solid-state image sensor. Such dark current noise increases substantially proportionally to the charge accumulating time (hereinafter called exposure time, as the accumulating time is substantially equal to the exposure time in case of a long exposure time) of the solid-state image sensor. In electronic still cameras with variable exposure time for the solid-state image sensor, the maximum exposure time of the solid-state image sensor is generally selected as long as possible within a range in which the dark current noise is not conspicuous. Under such setting, the dark current noise is not too conspicuous and tolerable as long as the exposure time T of the solid-state image sensor satisfies a condition T.ltoreq.T.sub.0, where T.sub.0 is the maximum exposure time (longest permissible exposure time) of the solid-state image sensor defined as explained above, but the dark current noise becomes too conspicuous and intolerable under a condition T&gt;T.sub.0. In case the ordinary gain A.sub.0 of the processing circuit for the output signal of the solid-state image sensor is increased to kA.sub.0 (k&gt;1; where K is a constant) for the purpose of sensitivity increase, the level of the dark current noise also increases k times. The dark current noise N.sub.0 at the maximum exposure time T.sub.0 of the solid-state image sensor and at the above-mentioned gain A.sub.0 can be represented, at a given temperature, by: EQU N.sub.0 =mA.sub.0 T.sub.0 ( 1)
wherein m is a constant. As explained above, the dark current noise up to the level N.sub.0 is tolerable. The dark current noise N.sub.1 at the maximum exposure time T.sub.0 and at an increased gain kA.sub.0 for sensitivity increase can be represented, at the given temperature, by: ##EQU1## wherein k is a constant larger than unity. Thus the S/N ratio of the image signal is reduced by a factor of 1/k, and the dark current noise becomes intolerable. This problem can be prevented by selecting a maximum exposure time permissible for the highest sensitivity and using that exposure time for all the values of sensitivity, but, in such case, the longer exposure time that should have been usable at a lower sensitivity becomes unusable, so that the range of exposure time becomes limited.